The development of a vaccine against the human immunodeficiency virus (HIV) is a crucial step in preventing further spread of acquired immunodeficiency syndrome (AIDS). For safety reasons, a whole virus vaccine may not be practical in the case of HIV. However, any subunit vaccine should contain immunodominant helper T-cell sites that could elicit helper T-cell immunity in response to exposure to the native antigen. Helper T cells are necessary for both B-cell activation by causing B-cell proliferation and differentiation into antibody-producing cells, and for induction of cytotoxic T cells. It should be noted that helper T-cells recognize distinct sites within the protein molecules rather than the entire protein antigen. Among these sites, a few usually elicit the bulk of the response and are, therefore, called "immunodominant." Hence, in order to elicit an effective cytotoxic or immunogenic response, the synthetic antigen should preferably include the immunodominant regions in the molecule. Furthermore, the synthetic antigenic peptide(s) should overcome the problem of MHC restriction often encountered with synthetic, fragment vaccines.